Engine systems may be configured with exhaust gas recirculation (EGR) systems via which at least a portion of the exhaust gas is recirculated to the engine intake. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, various temperature, pressure, oxygen, and humidity sensors coupled to the engine intake manifold and/or the exhaust manifold.
As such, EGR levels may be adjusted based on various conditions such as combustion air-fuel ratio and exhaust emissions levels. One example of such an adjustment is shown by Schilling et al in US 2013/0104544. Therein, during lean operation, an amount of EGR recirculated is increased so as to improve exhaust NOx emissions.
However, the inventors herein have identified potential issues with such an approach. As an example, engine control may be degraded due to incorrect EGR estimation. Specifically, during lean to very lean engine operation, there may be significant amounts of fresh air in the exhaust, and therefore in the EGR. If EGR delivery is estimated by a delta pressure sensor across the EGR valve, or across a fixed orifice of the EGR system (or an EGR MAF sensor), the sensor may incorrectly interpret the flow of fresh air as exhaust residuals, and EGR may be overestimated. As a result, adjustments to spark timing, throttle position, and other actuators based on this EGR estimate may be scheduled incorrectly leading to potential combustion and torque control issues. As another example, in engine systems where the EGR is estimated by an intake oxygen sensor, the fresh air may lead to an EGR measurement error that may be misinterpreted as reduced engine dilution, and EGR may be underestimated. In addition to incorrect engine control, this may also lead to OBD issues due to an EGR monitor noting a discrepancy between the expected EGR valve flow versus the EGR measured by the intake oxygen sensor. In still other systems, such as MAF systems, the fresh air through the EGR system may be unaccounted for, leading to aircharge calculation errors which may lead to fueling and torque errors.
The inventors herein have recognized that during lean operations where there is substantial fresh air in the exhaust, it may more efficient to not deliver any EGR rather than delivering an incorrect amount of EGR. In other words, the benefits of EGR on emissions and fuel efficiency may not outweigh the fuel penalty and performance penalty incurred due to incorrect EGR estimation and delivery. Thus in one example, some of the above issues may be at least partly addressed by a method for an engine comprising: while operating an engine with an air-fuel ratio adjusted to be leaner than stoichiometry, in response to exhaust air-fuel ratio being leaner than a threshold, closing an EGR valve.
As an example, during lean engine operation, a threshold may be set based on engine speed and load. The threshold may be based on a maximum amount of EGR error that may be tolerated. The tolerable EGR error may be used to calculate a degree of leanness that is acceptable. In response to the exhaust air-fuel ratio being leaner than the threshold, EGR delivery may be disabled by closing the EGR valve. Herein, the EGR may be a low pressure EGR and the EGR valve may be an EGR valve coupled in the low pressure EGR system. The valve may be maintained closed at least until engine operation has returned to be richer than the lean threshold, such as when engine operation at stoichiometry is resumed. Thereafter, EGR may be enabled.
In this way, issues associated with incorrect EGR estimation and misdiagnosis of an EGR system by an EGR monitor can be reduced. As such, this not only reduces combustion issues related to incorrect spark and torque control, but also reduces costs associated with failed EGR monitors. By removing the conditions that could cause false reading of EGR measurement, drivability and fuel economy are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.